The purpose of this proposal is to further elaborate on the nature of the association between Infantile Autism and the Fragile(X) (fra(X)) Syndrome. In a sample of 124 autistic males screened by our group, (Departments of Psychology and Human Genetics) thus far, 18 (14.5%) were found to be positive for the fra(X) site (200 will be screened by August, 1985). In addition, a high proportion of fra(X) cases were found to be either autistic or to show a number of characteristics similar to those displayed by autistic persons. This was confirmed in a controlled study using multivariate techniques. These observations suggested the hypothesis that fra(X) individuals, as a group, share a common set of behavioral characteristics with autistic individuals. Further, that this set of characteristics is less common among retarded individuals who are neither fra(X) nor autistic. Our specific aims, therefore, are as follows: 1. To screen 300 additional autistic males for the fra(X) site; 2a. To compare the behavioral, speech and language characteristics of fra(X) autistic, fra(X) non-autistic, and autistic clients in order to determine which shared characteristics differentiate them from Down Syndrome controls. 2b. To determine if fra(X) individuals share a common set of characteristics which distinguish them from the autistic and Down Syndrome groups. 3. Through longitudinal cohort analysis, to determine if fra(X) autistic and non-fra(X) autistic groups follow divergent or parallel behavioral development when followed on a yearly basis. A behavioral link established between the fra(X) syndrome and autism will be extremely important both in terms of genetic counselling and in eventually ascertaining the bio-medical bases of some autistic behaviors.